1. Field of the Invention
The present invention relates generally to a wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM). More particularly, the present invention relates to an apparatus and a method for acquiring initial synchronization when a receive signal is weak in an OFDM based wireless communication system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) technology can not only overcome frequency-selective fading using a simplified equalizer but also exhibits many features such as robustness against impulsive noise and high frequency utilization. As a result of those features, the OFDM technology is employed as core technology of wireless access systems such as the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standard and the 3GPP Long Term Evolution (LTE) standard. However, since signals are simultaneously transmitted over a plurality of subcarriers, time and frequency synchronization errors may greatly degrade the wireless access system performance by causing not only interference between neighbor signals but also interference between the subcarriers. The synchronization errors result in degradation of a handover performance of a terminal. Strict time synchronization and frequency synchronization are required between base stations in order to support seamless handover. For example, when the time synchronization between neighbor cells is poor in a Time Division Duplexing (TDD) based wireless communication system, downlink and uplink may overlap. As a result, the interference between the neighbor cells increases to notably deteriorate communication quality. To address this problem, Worldwide Interoperability for Microwave Access (WiMAX) systems define a base station time error and frequency error within 1% and 1 μs of the subcarrier spacing respectively.
Meanwhile, a method according to the related art utilizes a Global Positioning System (GPS) signal for the synchronization between the base stations. In the WiMAX system based on the IEEE 802.16 standard, each base station acquires the time synchronization and the frequency synchronization using the GPS signal. A GPS synchronizer used at the base station operates only in a region of good signal strength (−130 dBm or greater) received from a GPS satellite. However, a small base station installed indoors suffers from reception performance degradation because of the weak GPS signal strength. To overcome the reception performance degradation, a GPS antenna can be installed outdoors and then connected to the indoor base station using a Radio Frequency (RF) coaxial cable, which results in increased costs. Based on such limitations, to acquire the synchronization of the indoor small base station, consideration is given to a method for acquiring the synchronization based on a sync signal transmitted from the base station which is synchronized with the GPS signal.
The method for acquiring the synchronization based on the sync signal transmitted from the base station which is synchronized with the GPS signal works efficiently when the received signal strength is greater than a certain level (e.g., Signal-to-Noise Ratio (SNR)>−5 dB). In the indoor small base station, when the received signal strength is weak (e.g., SNR=−15 dB), caused by the signal attenuation of the outer wall and the inner wall of the building, noise increase in the cross correlation abruptly extends the initial synchronization acquisition time. When the SNR of the receive signal is high, the SNR decrease from the cross correlation is about 3 dB. When the SNR of the receive signal is quite low, the cross correlation reduces the SNR approximately by double in the dB region. For example, when the SNR of the receive signal is −15 dB, the SNR of the signal after the cross correlation becomes about −30 dB. Herein, to enhance the low SNR of the receive signal, it is necessary to suppress a noise signal by averaging the signal over tens or hundreds of frames. Accordingly, the time for synchronization acquisition increases.
Therefore, a need exists for an apparatus and method for reducing an initial synchronization acquisition time within an allowable time error and an allowable frequency error in a wireless communication system.